Tunneling field effect transistors (TFETs) for CMOS architectures and approaches to fabricating N-type and P-type TFETs

ABSTRACT

Tunneling field effect transistors (TFETs) for CMOS architectures and approaches to fabricating N-type and P-type TFETs are described. For example, a tunneling field effect transistor (TFET) includes a homojunction active region disposed above a substrate. The homojunction active region includes a relaxed Ge or GeSn body having an undoped channel region therein. The homojunction active region also includes doped source and drain regions disposed in the relaxed Ge or GeSn body, on either side of the channel region. The TFET also includes a gate stack disposed on the channel region, between the source and drain regions. The gate stack includes a gate dielectric portion and gate electrode portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/209,552, filed on Jul. 13, 2016, which is a divisional of U.S. patentapplication Ser. No. 14/521,200, filed on Oct. 22, 2014, now U.S. Pat.No. 9,412,872, issued on Aug. 9, 2016, which is a divisional of U.S.patent application Ser. No. 13/678,867, filed on Nov. 16, 2012, now U.S.Pat. No. 8,890,120, issued on Nov. 18, 2014, the entire contents ofwhich are hereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the invention are in the field of semiconductor devicesand, in particular, tunneling field effect transistors (TFETs) for CMOSarchitectures and approaches to fabricating N-type and P-type TFETs.

BACKGROUND

For the past several decades, the scaling of features in integratedcircuits has been a driving force behind an ever-growing semiconductorindustry. Scaling to smaller and smaller features enables increaseddensities of functional units on the limited real estate ofsemiconductor chips. For example, shrinking transistor size allows forthe incorporation of an increased number of memory devices on a chip,leading to the fabrication of products with increased capacity. Thedrive for ever-more capacity, however, is not without issue. Thenecessity to optimize the performance of each device becomesincreasingly significant.

In the manufacture of integrated circuit devices, multi-gatetransistors, such as tri-gate transistors, have become more prevalent asdevice dimensions continue to scale down. In conventional processes,tri-gate transistors are generally fabricated on either bulk siliconsubstrates or silicon-on-insulator substrates. In some instances, bulksilicon substrates are preferred due to their lower cost and becausethey enable a less complicated tri-gate fabrication process. On bulksilicon substrates, however, the fabrication process for tri-gatetransistors often encounters problems when aligning the bottom of themetal gate electrode with the source and drain extension tips at thebottom of the transistor body (i.e., the “fin”). When the tri-gatetransistor is formed on a bulk substrate, proper alignment is needed foroptimal gate control and to reduce short-channel effects. For instance,if the source and drain extension tips are deeper than the metal gateelectrode, punch-through may occur. Alternately, if the metal gateelectrode is deeper than the source and drain extension tips, the resultmay be an unwanted gate capacitance parasitics.

Many different techniques have been attempted to reduce junction leakageof transistors. However, significant improvements are still needed inthe area of junction leakage suppression.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates angled views of (a) a portion of a homojunction TFETdevice having an unstrained Ge or GeSn narrow body, in accordance withan embodiment of the present invention, and (c) a portion of ahetero-junction TFET device having an unstrained narrow source/channeljunction, in accordance with an embodiment of the present invention. In(b), the leading band edges are shown for a relaxed 5 nm Ge double gatedevice, corresponding to (a). The leading edges for the band alignmentfor the structure of (c) are shown in (d).

FIG. 2A illustrates an angled view of a portion of a planar biaxialtensile strained Ge or GeSn homojunction TFET device, in accordance withan embodiment of the present invention.

FIG. 2B illustrates an angled and partially cross-sectioned view of aportion of a suspended nanowire or nanoribbon Ge or GeSn homojunctionbased TFET device, in accordance with an embodiment of the presentinvention.

FIG. 2C illustrates an angled view of a portion of a tri-gate or finfetGe homojunction based TFET device, in accordance with an embodiment ofthe present invention.

FIG. 3A illustrates an angled view of a portion of a vertical TFETdevice having a tensile strained Ge region, in accordance with anembodiment of the present invention.

FIG. 3B illustrates an angled view of a portion of another vertical TFETdevice having a tensile strained Ge region, in accordance with anembodiment of the present invention.

FIG. 3C illustrates an angled view of a portion of yet another verticalTFET device having a tensile strained Ge region, in accordance with anembodiment of the present invention.

FIG. 4 illustrates an angled view of a portion of a vertical TFET devicehaving a tensile strained Ge_(1-y)Sn_(y) region, in accordance with anembodiment of the present invention.

FIG. 5 is a band energy diagram 500 for bulk relaxed Ge at a temperatureof approximately 300K, in accordance with an embodiment of the presentinvention.

FIG. 6 is a Table of electron masses along different confinementorientations for a finfet device for four L-valleys, in accordance withan embodiment of the present invention.

FIG. 7 is a plot of simulated drain current (ID) as a function of gatevoltage (VG) for N- and P-type unstrained Ge devices, in accordance withan embodiment of the present invention.

FIG. 8 is a plot of simulated energy (meV) as a function of biaxialstress (MPa) bulk Ge devices, in accordance with an embodiment of thepresent invention.

FIG. 9A is a plot of simulated drain current (ID) as a function of gatevoltage (VG) for N- and P-type strained and unstrained Ge devices, inaccordance with an embodiment of the present invention.

FIG. 9B is a plot of simulated drain current (ID) as a function of gatevoltage (VG) in P-type strained Ge or III-V material devices, inaccordance with an embodiment of the present invention.

FIG. 10A is a plot 1000A showing the direct and indirect band gap inGeSn versus Sn content calculated using the Jaros' band offset theory,in accordance with an embodiment of the present invention.

FIG. 10B is a plot 1000B depicting the transition for aGe_(1-x-y)Si_(x)Sn_(y) ternary alloy, in accordance with an embodimentof the present invention.

FIG. 11A is a plot depicting stress simulation of the structure shown inFIG. 3A for varying wire dimensions, in accordance with an embodiment ofthe present invention.

FIG. 11B is a plot depicting stress simulation of the structure shown inFIG. 3B, in accordance with an embodiment of the present invention.

FIG. 11C is a plot depicting stress simulation of the structure shown inFIG. 3C, in accordance with an embodiment of the present invention.

FIG. 12 illustrates a computing device in accordance with oneimplementation of the invention.

DESCRIPTION OF THE EMBODIMENTS

Tunneling field effect transistors (TFETs) for CMOS architectures andapproaches to fabricating N-type and P-type TFETs are described. In thefollowing description, numerous specific details are set forth, such asspecific integration and material regimes, in order to provide athorough understanding of embodiments of the present invention. It willbe apparent to one skilled in the art that embodiments of the presentinvention may be practiced without these specific details. In otherinstances, well-known features, such as integrated circuit designlayouts, are not described in detail in order to not unnecessarilyobscure embodiments of the present invention. Furthermore, it is to beunderstood that the various embodiments shown in the Figures areillustrative representations and are not necessarily drawn to scale.

One or more embodiments described herein target approaches to, and theresulting devices from, using an indirect bandgap to direct bandgaptransition for complementary N-type and P-type TFET devices. In morespecific embodiments, the TFET devices are fabricated from Group IVmaterials. The devices may have applications in logic architectures, andin lower power device architectures. One or more embodiments aredirected to achieving high performance N-type and P-type TFET devices byusing indirect to direct bandgap transitions in group IV materials.Methods and structures to engineer such devices are described herein. Inone embodiment, TFETs are used to achieve steeper subthreshold slope(SS) versus a corresponding metal oxide semiconductor field effecttransistor (MOSFET) with a thermal limit of approximately 60 mV/decade.Generally, embodiments described herein may be suitable for highperformance or scaled transistors for logic devices having low powerapplications.

To provide a background context, due to the presence of direct band gapsand a wide variety of hetero-structure band alignments, group III-Vmaterial based TFETs should offer high drive current and low SS. A SSless than 60 mV/decade has been achieved for a group III-V materialhetero-structure pocket N-type TFET. With further device optimization ofequivalent oxide thickness (EOT), body scaling, and barrier engineering,the group III-V material N-type TFET is expected to outperform groupIII-V material MOSFETs at a low target VCC, e.g., a VCC of approximately0.3V. However, the low density of conduction band states in group III-Vmaterials may present a fundamental limitation on achieving both a lowSS and high on current (I_(ON)) in P-type TFETs based on group III-Vmaterials.

Furthermore, the I_(ON) current in TFETs fabricated in or fromtechnologically important group IV materials, such as silicon (Si),germanium (Ge), or silicon germanium (SiGe), may be limited by a largerbandgap (e.g., 1.12 eV in Si) and/or a low indirect band gap tunnelingcurrent. In Si and Ge, the top valence bands are at the gamma point,while the lowest conduction bands are at the delta point in Si and Lpoint in Ge. The tunneling between the conduction band and the valenceband at the source/channel junction is enabled by a phonon-assistedtwo-step process. The process typically has a low probability which maylead to a low I_(ON) for TFETs based on indirect bandgap materials. Forexample, in the best performing Si/SiGe hetero-structure TFET theexperimentally achieved I_(ON) is approximately 40 nA/micron at 1V gateoverdrive, which is approximately 25 times lower than the abovedescribed I_(ON) for group III-V material devices at 0.3V gateoverdrive. A corresponding high I_(ON) for Si, Ge, or SiGe based TFETshas not yet been achieved. Accordingly, one or more embodimentsdescribed herein target approaches to fabricating high performanceN-type and P-type TFETs with low SS and high I_(ON) in the same materialsystem.

In an embodiment, band engineering of a band structure of group IVmaterials, and their alloys, is used to achieve an indirectbandgap-to-direct bandgap transition for enabling N-type and P-type TFETdevices in the same material. The group IV materials do not suffer fromthe low conduction density of states. Furthermore, with the engineereddirect band gap, a high I_(ON) and low SS can be achieved in both N-typeand P-type TFETs fabricated in a same material. In specific embodiments,both unstrained and strained Ge-based or GeSn-based N-type and P-typeTFETs are described.

In a first aspect, one or more embodiments described herein are directedto methods of achieving an indirect-direct bandgap transition for use inTFETs. For example, in one embodiment, wafer orientation and conductionband non-parabolicity effect is used to increase the conduction bandgamma valley mass under confinement in a thin body fin field effecttransistor (finfet) or nanowire Ge or germanium tin (GeSn) TFET. Such adevice provides a conduction band gamma valley energy as the lowestconduction band edge to achieve the direct bandgap. In anotherembodiment, tensile strain in Ge, GeSn, or silicon germanium tin(SiGeSn) is used to achieve a direct bandgap. In another embodiment,alloying of Ge with Sn in relaxed GeSn or SiGeSn is used to achieve adirect bandgap. Specific embodiments of the above approaches aredescribed below in association with FIGS. 5-11.

In a second aspect, one or more embodiments described herein aredirected to structures for TFET devices which utilize a direct bandgaptransition. For example, in one embodiment, a device is based on anunstrained Ge or GeSn narrow body homojunction TFET or an unstrained Geor GeSn narrow source/channel junction hetero-structure TFET usingfinfet or nanowire/nanoribbon device geometries. The confinement leadsto the indirect-to-direct bandgap transition at or below approximately 5nm body thickness in finfet, or in a wide rectangular nanoribbon or asquare nanowire. These devices are fabricated to have (100), (010) or(001) orientations at the device surfaces. The direct bandgap materialis disposed either throughout the device, or in the source/channeljunction of the device. In the drain/body of the hetero-structuredevice, a lattice-matched direct wide bandgap material is used tominimize the off state current (I_(OFF)) of the device. In anotherembodiment, a finfet or nanowire is based on an unstrainedGe_(1-x)Sn_(x) homojunction TFET with the Sn content x>6%, although therequirement to have a narrow body to achieve the direct band gap may berelaxed in this case. Examples of the immediately above describeddevices are illustrated in FIG. 1, a description of which follows.

Generally, FIG. 1 illustrates angled views of (a) a portion 100A of aTFET device having an unstrained Ge or GeSn narrow body, e.g., at orless than approximately 5 nm dimension finfet or squarenanowire/nanoribbon homojunction, and (c) a portion 100C of a TFETdevice having an unstrained narrow source/channel junction, e.g., at orless than approximately 5 nm dimension. The direct bandgap material iseither throughout the device in (a), or in the source/channel junctionin (c). In (b) of FIG. 1, the leading band edges are shown for a relaxed5 nm Ge double gate device. To achieve the direct bandgap at the largestminimum body dimension, the confinement direction in the correspondingfinfet is <100> (or <010>, or <001>), and the surface orientations are(100) (or (010), or (001)) in a wire/ribbon based device. In thehetero-structure in (c), i.e., portion 100C, the lattice-matched directwide bandgap material is used to minimize the I_(OFF) in the drain/bodyof the device. In an exemplary embodiment, an example choice for ahetero-structure in (c) is a Ge narrow source/channel junction andlattice matched relaxed III-V material GaAs or Ga_(0.5)In_(0.5)P in thebody and in the drain. The leading edges for the band alignment for thestructure of (c) are shown in (d).

More specifically, referring again to FIG. 1, the portion 100A of a TFETdevice includes an undoped and unstrained Ge or GeSn narrow body 102having a thickness 104. Source (Na/Nd) 106 and drain (Nd/Na) 108 regionsare doped regions formed in the same Ge or GeSn material. The portion100A may be used to fabricate a narrow body homojunction Ge or GeSnN-type or P-type TFET homojunction-based devices. In (b) the band energy(eV) as a function of the distance x along the structure 100A isprovided for a device with 5 nm body dimension. The portion 100C of aTFET device includes an undoped and unstrained Ge or GeSn narrow bodyfirst portion 152 having a thickness 154. A lattice matched narrow bodysecond portion 153 is also included and may be fabricated from a latticematched Group III-V material as described above. Source (Na/Nd) 156region is formed as a doped region of the Ge or GeSn material of 152having a thickness 157, while drain (Nd/Na) 158 region is formed as adoped region of the lattice matched III-V material. The portion 100C maybe used to fabricate a narrow source/channel junction for Ge or GeSnN-type or P-type TFET hetero-junction-based devices. In (d) the bandenergy (eV) as a function of the distance x along the structure 100C isprovided for a device with 5 nm body dimension.

In another example of structures for TFET devices which utilize a directbandgap transition, in an embodiment, a TFET device is based on a planarbiaxial tensile strained Ge homojunction structure, with Ge strainobtained from a Ge film grown pseudomorphically on a relaxed substratehaving a larger lattice constant. In a specific embodiment, possibleselections for the substrate include, but are not limited to,Ge_(1-x)Sn_(x) and In_(x)Ga_(1-x)As. For example, the growth of biaxialtensile Ge and GeSn on In_(x)Ga_(1-x)As relaxed buffers layers mayprovide a suitable approach. However, in an embodiment, approximately12.5% of Sn or approximately 30% of indium (In) is used to fabricate thedirect bandgap material in an approximately 5 nm body dimension Ge-basedTFET. In another embodiment, a planar biaxial tensile strainedGe_(1-y)Sn_(y) with less than approximately 6% of Sn is used in ahomojunction TFET device, with Ge_(1-y)Sn_(y) strain obtained from aGe_(1-y)Sn_(y) film grown pseudomorphically on a relaxed substratehaving a larger lattice constant. In a specific such embodiment,possibilities for the substrate include, but are not limited to,Ge_(1-x)Sn_(x) and In_(x)Ga_(1-x)As. Examples of the immediately abovedescribed devices are illustrated in FIG. 2A, a description of whichfollows.

Generally, FIG. 2A illustrates an angled view of a portion 200A of aplanar biaxial tensile strained Ge or GeSn homojunction TFET device, inaccordance with an embodiment of the present invention. In oneembodiment, strain for the device is derived from a layer grownpseudomorphically on a relaxed substrate with a larger lattice constant.Possibilities for the substrate include, but are not limited to,Ge_(1-x)Sn_(x) and In_(x)Ga_(1-x)As having a larger lattice constantsthan a corresponding active layer. More specifically, referring again toFIG. 2A, the portion 200A of a TFET device includes an active layer 204Adisposed on a substrate 202A. The substrate 202A is a relaxed bufferhaving a lattice constant greater than the lattice constant of theactive layer 204A. An undoped body 206A having a thickness 207A isdisposed between doped source (Na/Nd) region 208A and doped drain(Nd/Na) region 210A. A gate electrode 212A and gate dielectric 214Astack is formed above the undoped body 206A. In an embodiment, thestructure 200A is used to fabricate a planar Ge or GeSn N-type or P-TypeTFET having biaxial tensile stress.

In another example of structures for TFET devices which utilize a directbandgap transition, in an embodiment, a TFET device is based on asuspended nanowire or nanoribbon Ge homojunction. In a specificembodiment, a TFET device is undercut in a channel region of a planarbiaxial tensile strained Ge film, with Ge strain obtained from Ge filmgrown pseudomorphically on a relaxed substrate having a larger latticeconstant. Possibilities for the substrate include, but are not limitedto, Ge_(1-x)Sn_(x) or In_(x)Ga_(1-x)As. In a specific embodiment, aconcentration of approximately 12.5% Sn or 30% In is used to produce adirect bandgap material for an approximately 5 nm body dimension GeTFET.

Generally, FIG. 2B illustrates an angled and partially cross-sectionedview of a portion 200B of a suspended nanowire or nanoribbon Gehomojunction based TFET device, in accordance with an embodiment of thepresent invention. In one embodiment, the device is fabricated byundercutting in a channel region of a planar biaxial tensile strained Gefilm. Ge strain may be obtained from a Ge film grown pseudomorphicallyon a relaxed substrate having a larger lattice constant. Possibilitiesfor the substrate include, but are not limited to, Ge_(1-x)Sn_(x) orIn_(x)Ga_(1-x)As. In an embodiment, such a structure enables a directbandgap due to a combined effect of confinement and stress. Morespecifically, referring again to FIG. 2B, the portion 200B of a TFETdevice includes an active layer 204B disposed on a substrate 202B. Thesubstrate 202B is a relaxed wide buffer having a lattice constantgreater than the lattice constant of the active layer 204B. The activelayer 204B is undercut in region 250B to provide an undoped body 206Bhaving a thickness 207B disposed between doped source (Na/Nd) region208B and doped drain (Nd/Na) region 210B. A gate electrode 212B and gatedielectric 214B stack is formed to wrap around the undoped body 206B. Inan embodiment, the structure 200B is used to fabricate a nanowire ornanoribbon Ge or GeSn N-type or P-Type TFET having biaxial tensilestress.

In another example of structures for TFET devices which utilize a directbandgap transition, in an embodiment, a TFET device is based on atri-gate or finfet Ge homojunction. In one embodiment, the device isfabricated by cutting a layer region into a fin in a channel region of aplanar biaxial tensile strained Ge film. In a specific embodiment, Gestrain is obtained from a Ge film grown pseudomorphically on a relaxedsubstrate having a larger lattice constant. Possibilities for thesubstrate include, but are not limited to, Ge_(1-x)Sn_(x) orIn_(x)Ga_(1-x)As. Such a structure may enable a direct bandgap due to acombined effect of confinement and uniaxial tensile stress. In oneembodiment, the uniaxial tensile stress and the transport directions arealong one of the principal crystal orientations of <100>, <010>, or<001>.

Generally, FIG. 2C illustrates an angled view of a portion 200C of atri-gate or finfet Ge homojunction based TFET device, in accordance withan embodiment of the present invention. In one embodiment, the device isfabricated by cutting a layer region into a fin a the channel region ofplanar biaxial tensile strained Ge film. Ge strain may be derived from aGe film grown pseudomorphically on a relaxed substrate having a largerlattice constant. In an embodiment, possible choices for the substrateinclude, but are not limited to, Ge_(1-x)Sn_(x) or In_(x)Ga_(1-x)As.More specifically, referring again to FIG. 2C, the portion 200C of aTFET device includes a tensile strained active layer 204C disposed on asubstrate 202C. The substrate 202C is a relaxed wide buffer having alattice constant greater than the lattice constant of the active layer204C. The active layer 204C is patterned to have a fin geometry 250C toprovide an undoped body 206C having a thickness 207C disposed betweendoped source (Na/Nd) region 208C and doped drain (Nd/Na) region 210C. Agate electrode 212C and gate dielectric 214C stack is formed on the topand exposed sides of the undoped body 206C. In an embodiment, thestructure 200C is used to fabricate a trigate or finfet Ge or GeSn basedN-type or P-Type TFET having uniaxial tensile stress. In a specificembodiment, the device has a transport direction along a crystalorientations of <100>, <010>, or <001>.

In another example of structures for TFET devices which utilize a directband gap transition, in an embodiment, a TFET device is based on avertical thin body with a biaxial tensile strained Ge region used as asource, or a source/channel junction. In one such embodiment, fordimension considerations, the Ge region has a vertical dimensionapproximately in the range of 2-4 nanometers. There are a number ofpossible approaches to achieving a high tensile strain for fabricating adirect gap source region with Ge, examples of which are described belowin association with FIGS. 3A-3C. Although not necessarily depicted,other options for fabricating strained Ge source materials include, butare not limited to, embedding the Ge inside a relaxed GeSn or tensilestrained SiGe structure.

In a first example, FIG. 3A illustrates an angled view of a portion 300Aof a vertical TFET device having a tensile strained Ge region, inaccordance with an embodiment of the present invention. Referring toFIG. 3A, the TFET device is formed above a virtual substrate 302A formedabove a substrate 301A. A germanium source region 304A is included andhas tensile strain. Above the germanium source region 304A is a channelregion 306A and drain region 308A. In one embodiment, the channel region306A and drain region 308B are formed from a same material, such asGeSn, as depicted in FIG. 3A. In an embodiment, the virtual substrate302A includes a relaxed layer such as but not limited to relaxed InGaAsor relaxed GeSn. The corresponding indium or tin percent may be selectedto tune the strain in the Ge layer 304A. For example, an Sn percentageof approximately 14% or an In percentage of approximately 30% may beused to provide approximately 2.5 GPa of biaxial stress in the Ge layer304A if the Ge layer 304A is deposited as a blanket film. It is to beunderstood, however, that due to relaxation caused by forming a verticalwire, higher mismatches may be needed to achieve highly strained Ge inthe final device. In an embodiment, by using a square layout, asdepicted in FIG. 3A, the stress can be made more biaxial. Although notshown, it is to be understood that a gate stack, including a gatedielectric layer and a gate electrode layer, is formed to at leastpartially, if not completely, surround channel region 306A.

In a second example, FIG. 3B illustrates an angled view of a portion300B of another vertical TFET device having a tensile strained Geregion, in accordance with an embodiment of the present invention.Referring to FIG. 3B, the TFET device is formed above a strained layer302B formed above a virtual substrate 301B. A germanium source region304B is included and has tensile strain. Above the germanium sourceregion 304B is a channel region 306B and drain region 308B. In oneembodiment, the strained layer 302B, the channel region 306B, and thedrain region 308B are formed from a same material, such as strainedGeSn, as depicted in FIG. 3B. In an embodiment, the virtual substrate301B is a relaxed Ge virtual substrate. In an embodiment, the GeSn layer302B is formed as a compressively strained layer. The Ge layer 304B isdeposited as a strain-free layer and then capped with compressivelystrained GeSn 306B/308B. In an embodiment, upon patterning such amaterial stack into a wire, the elastic relaxation of the GeSn stretchesthe Ge (layer 304B) causing it to be tensile. Although not shown, it isto be understood that a gate stack, including a gate dielectric layerand a gate electrode layer, is formed to at least partially, if notcompletely, surround channel region 306B.

In a third example, FIG. 3C illustrates an angled view of a portion 300Cof another vertical TFET device having a tensile strained Ge region, inaccordance with an embodiment of the present invention. Referring toFIG. 3C, the TFET device is formed above a virtual substrate 302C. Agermanium source region 304C is included and has tensile strain. Abovethe germanium source region 30CB is a channel region 306C and drainregion 308C. In one embodiment, the channel region 306C and the drainregion 308C are formed from a same material, such as strained GeSn, asdepicted in FIG. 3C. In an embodiment, the virtual substrate 302C is arelaxed GeSn virtual substrate, e.g., having approximately 14% Sn. TheGe layer 304C is a tensile strained Ge, while the GeSn region 306C/308Cis compressively strained and has a composition of approximately 28% Sn.It is to be understood that other materials with similar latticeconstants may be used instead of GeSn virtual substrate 302C. When thestructure 300C is formed into a wire, in an embodiment, the compressiveGeSn 306C/308C aids in retaining tensile strain in the Ge layer 304C atthe interface. Although not shown, it is to be understood that a gatestack, including a gate dielectric layer and a gate electrode layer, isformed to at least partially, if not completely, surround channel region306C.

In another example of structures for TFET devices which utilize a directband gap transition, in an embodiment, a TFET device is based on avertical thin body with a biaxial tensile strained Ge_(1-y)Sn_(y) regionused as a source, or a source/channel junction. In one such embodiment,for dimensional considerations, the Ge_(1-y)Sn_(y) region has a verticaldimension approximately in the range of 2-4 nanometers. There are anumber of possible approaches to achieving a high tensile strain forfabricating a direct gap source region with Ge_(1-y)Sn_(y), an exampleof which is described below in association with FIG. 4.

FIG. 4 illustrates an angled view of a portion 400 of a vertical TFETdevice having a tensile strained Ge_(1-y)Sn_(y) region, in accordancewith an embodiment of the present invention. Referring to FIG. 4, theTFET device is formed above a virtual substrate 402 formed above asubstrate 401. A germanium tin (GeSn) source region 404 is included andhas tensile strain. Above the GeSn source region 404 is a channel region406 and drain region 408. In one embodiment, the channel region 406 anddrain region 408 are formed from a same material, such as GeSn, asdepicted in FIG. 4. In an embodiment, the virtual substrate 402 includesa relaxed layer such as but not limited to relaxed InGaAs or relaxedGeSn. The corresponding indium or tin percent may be selected to tunethe strain in the GeSn layer 404. Due to relaxation caused by forming avertical wire, higher mismatches may be needed to achieve highlystrained GeSn in the final device. In an embodiment, by using a squarelayout, as depicted in FIG. 4, the stress can be made more biaxial.Although not shown, it is to be understood that a gate stack, includinga gate dielectric layer and a gate electrode layer, is formed to atleast partially, if not completely, surround channel region 406.

In an aspect, then, approaches to achieving an indirect-to-direct bandgap transition for fabricating P-type and/or N-type TFETs include theuse of wafer orientation and conduction band non-parabolicity effects toincrease the conduction band gamma valley mass under confinement in athin body finfet or nanowire Ge or GeSn TFET. Such approaches provide aconduction band gamma valley energy as the lowest conduction band edgein order to realize a direct band gap.

As an example, a conduction band edge at a gamma point is parabolic inzinc blende materials, but away from the band edge it exhibitsnon-parabolicity based on equation (1):m _(Γ) =m _(Γ0)(1+α∈)  (1).Materials with smaller bandgap exhibit larger non-parabolicity. Thenon-parabolicity constant α depends on the bandgap and effective mass inthe material, as shown in equation (2):

$\begin{matrix}{\alpha = {\frac{\left( {1 - \frac{m^{*}}{m_{0}}} \right)^{2}}{E_{gap}}.}} & (2)\end{matrix}$

For example, for germanium (Ge) gamma point effective mass m* is 0.04m₀, the direct bandgap is 0.8 eV, and the non-parabolicity constant α is1.15 eV⁻¹. For L-valley edges the non-parabolicity constant issignificantly smaller at 0.3 eV⁻¹. In the relaxed Ge bulk bandstructure, the gamma valley is 0.14 eV above the L-valley, as shown inFIG. 5. For such an indirect band gap material band structure, theballistic current is vanishingly small, and the allowed tunnelingprocesses are phonon-assisted which have a low probability and lead to alow ON current in a relaxed thick body Ge TFET.

FIG. 5 is a band energy diagram 500 for bulk relaxed Ge at a temperatureof approximately 300K, in accordance with an embodiment of the presentinvention. Referring to plot 500, the band gap is indirect in that thelowest in energy conduction bands are at L-points, and the top valencebands are at gamma points. The band-to-band tunneling process at thesource/channel junction is a phonon assisted two-step process with lowprobability which leads to a low I_(ON) in TFETs based on indirectbandgap materials.

In a quantum confined structure, the energy ∈ corresponds to the shiftof the band edge energy due to confinement. With stronger confinement innarrow structures, the band energy increases and, therefore, the gammavalley mass increases with a smaller structure size. The L-valley massincreases less with stronger confinement, and gamma valley becomes thelowest conduction band edge at a narrow structure size. To achieve thedirect bandgap at the largest minimum structure size, in an embodiment,an optimum wafer orientation for the confinement is used. For example,in a specific embodiment, in bulk Ge there are 8 L-valleys with heavylongitudinal mass ml=1.56 m₀ along the <111>, <11-1>, <−111>, and <1-11>directions (and along the corresponding opposite directions), and thelight transverse mass mt=0.082 m₀ along perpendicular directions. The<100> confinement direction in a finfet, or (100) confinement plane in awire may provide the lightest mass for all L-valleys and, therefore,maximally raise the corresponding energies under confinement. Suchraising of the corresponding energies under confinement may allow anindirect to direct transition to be achieved at the largest minimumstructure size.

In an exemplary embodiment, FIG. 6 is a Table 600 of electron massesalong different confinement orientations for a finfet device for fourL-valleys. Referring to Table 600, conduction band masses (in units ofelectron mass) in bulk Ge along <001>, <111>, and <1-10> confinementdirections are provided for the L-valleys. The gamma valley is isotropicwith a mass of 0.04 m₀ in bulk Ge.

With an increased confinement in narrow body TFET devices, thecorresponding gamma mass may increase due to the non-parabolicity effectand, at an approximately 5 nanometer body, may become the lowestconduction band leading to the direct bandgap in Ge. In such asituation, a direct ballistic tunneling current may provide acompetitive high I_(ON) and low SS both in the N-type and P-type Geunstrained (100) TFETs, as simulated in FIG. 7.

FIG. 7 is a plot 700 of simulated drain current (ID) as a function ofgate voltage (VG) for N- and P-type Ge devices, in accordance with anembodiment of the present invention. Referring to FIG. 7, simulatedballistic current in a narrow 5 nm body double-gate relaxed (100) Gehomojunction N-Type or P-type TFET is plotted as a function of gateoverdrive. For the simulation, Lgate=40 nm, EOT=1 nm, source/drainextensions are 20 nm, source/drain dopings are 5e19 cm⁻³. Relaxed Gebecomes a direct bandgap material due to the narrow body confinementleading to competitive ON current of 1 μA/μ and a min SS of 12 mV/dec innTFET or 15 mV/dec in pTFET. The simulation involves the NEGF quantumtransport method and the sp3s*d5_SO tight-binding band structure modelimplemented in an OMEN simulator. It is to be understood that, inaccordance with an embodiment of the present invention, further increaseof I_(ON) can be obtained by using the hetero-structure design with thenarrow body direct bandgap material in the source or in thesource/channel junction.

In another aspect, approaches to achieving an indirect-to-direct bandgap transition for fabricating P-type and/or N-type TFETs include theuse of tensile strain in Ge, GeSn, or SiGeSn to achieve the direct bandgap.

As an example, a tensile biaxial stress or tensile uniaxial stress alongthe principal crystal orientations <100>, <010>, <001> in Ge, GeSn,SiGeSn or a combination of these tensile stresses may be used to achievethe direct bandgap. In an embodiment, the applied mechanical stressbreaks crystal symmetries, and splits band degeneracies. In adeformation potential theory, the band edge shifts with applied stressare linearly proportional to strains having deformation potentials asproportionality coefficients. For example, in a specific embodiment,under an applied tensile biaxial strain in bulk Ge, the gamma valleybecomes the lowest band edge above 2 GPa stress as shown in FIG. 8. Thecorresponding band gap also narrows with stress.

FIG. 8 is a plot 800 of simulated energy (meV) as a function of biaxialstress (MPa) bulk Ge devices, in accordance with an embodiment of thepresent invention. Referring to plot 800, band gap narrowing andcorresponding energy difference between a conduction band gamma valleyedge and the closest conduction band edge of other valleys as a functionof the applied biaxial stress in bulk Ge are shown. The calibrated modelused applies the deformation potential theory of Bir and Pikus. In aspecific embodiment, above approximately 2 GPa of tensile biaxial stressGe becomes direct and can be used to enhance performance of N-type andP-type TFETs.

The above described approach involves use of tensile stress to achieve adirect bandgap material in Ge, GeSn, or SiGeSn in order to engineer highI_(ON) and low SS in group IV materials. For example, in an embodiment,under an application of a 2.5 GPa tensile biaxial stress in narrow 5 nmbody homojunction Ge—N-type and/or and P-type TFETs, the I_(ON) atVG=VCC is increased by greater than approximately 5× in both N-type andP-type Ge TFETs, as shown simulated in FIG. 9A. In one such embodiment,approximately twice the amount of uniaxial tensile stress is needed toachieve the direct bandgap in Ge. However, less hydrostatic tensilestress may be needed to achieve the direct bandgap in Ge. The Ge P-typeTFET with the direct bandgap due to the combined effect of confinementand stress shows an advantage of approximately 3× lower SS than in thesimulated 5 nm body III-V material P-type TFET, as depicted in FIG. 9B.

FIG. 9A is a plot 900A of simulated drain current (ID) as a function ofgate voltage (VG) for N- and P-type Ge devices, in accordance with anembodiment of the present invention. Referring to plot 900A, simulatedballistic drain current is observed in a narrow (100) 5 nm bodydouble-gate relaxed and under 2.5 GPa tensile biaxial strain Gehomojunction N-type or P-type TFET as a function of gate overdrive. Forthe simulation, Lgate=40 nm, EOT=1 nm, source/drain extensions are 20nm, source/drain dopings are 5e19 cm⁻³. Strained Ge is a direct bandgapmaterial leading to ON current gains of greater than approximately 5×over the relaxed material at VG=VCC, while maintaining a low minimum SSof 19 mV/dec in the N-type TFET and 15 mV/dec in the P-type TFET. In anembodiment, further increase of I_(ON) can be achieved by using thehetero-structure design with the narrow body strained direct bandgapmaterial in the source.

FIG. 9B is a plot 900B of simulated drain current (ID) as a function ofgate voltage (VG) P-type Ge or III-V material devices, in accordancewith an embodiment of the present invention. Referring to plot 900B,simulated ballistic drain current is shown for the narrow (100) 5 nmbody double-gate under 2.5 GPa tensile biaxial strain for a Gehomojunction P-type TFET and for a hetero-junction In_(0.53)Ga_(0.47)AsP-type TFET with 4 nm InAs pocket at the source as a function of gateoverdrive. For the simulation, Lgate=40 nm, EOT=1 nm, source/drainextensions are 20 nm, source/drain dopings are 5e19 cm⁻³. As depicted inFIG. 9B, and in accordance with an embodiment of the present invention,the Ge-based P-type TFET shows approximately 3× lowering of SS ascompared with the III-V material-based P-type TFET.

In the above described approach to achieving a direct band gap in TFETs,a tensile stress in the finfet or a nanowire is used. The tensile stresseffect can be combined with a narrow body confinement effect to maximizethe TFET performance. Such an approach can be implemented in planarbiaxially strained Ge, GeSn, SiGeSn pseudomorphic films or in narrowbody Ge homojunction TFET, or narrow body Ge source—GeSnhetero-structures. In one such embodiment, indirect band gap to directbandgap transitions due to applied tensile stress in GeSn for Sn contentless than approximately 6% can be used.

In another aspect, approaches to achieving an indirect-to-direct bandgap transition for fabricating P-type and/or N-type TFETs include theuse of alloying of Ge with Sn in relaxed GeSn or SiGeSn to achieve adirect band gap.

In an example, it is to be understood that Ge is an indirect bandgapmaterial, while Sn is a metal. During alloying Ge with Sn, the resultingGeSn undergoes an indirect-band gap-direct-band gap transition for Snconcentrations above approximately 6%-10%. In accordance with anembodiment of the present invention, the direct and indirect bandgap inGeSn vs Sn content calculated using the Jaros' band offset theory areshown in FIG. 10A. The transition for the Ge_(1-x-y)Si_(x)Sn_(y) ternaryalloy is shown in FIG. 10B. Referring to FIGS. 10A and 10B, band gaps ofGe_(1-z)Sn_(z) at L, gamma, and X conduction band valleys versus Sncomposition of z shows the indirect-to direct bandgap transition above6% of Sn. The lowest (either direct or indirect) band gap of relaxedGe_(1-x-y)Si_(x)Sn_(y) alloys may be calculated by empiricalpseudopotential method. For such an approach, the alloy GeSn or SiGeSnis used to provide a direct band gap in TFETs. The alloy effect may becombined with a narrow body confinement effect, and tensile stresseffect to maximize the TFET performance. The approach may be implementedin relaxed GeSn, SiGeSn films in narrow body homojunction TFET, ornarrow body GeSn/SiGeSn source—GeSn/Ge/SiGe hetero-structures.

In another aspect, approaches are provided to achieve stress in TFETdevices which utilize direct band gap transitions under an appliedstress. As an example, FIG. 11A is a plot 1100A depicting stresssimulation of the structure shown in FIG. 3A for varying wiredimensions, in accordance with an embodiment of the present invention.Referring to plot 1100A, the two in plane components of the stress areplotted for the case where the deposited Ge film has 2% mismatch strainwith the virtual substrate. For smaller size wires mismatch greater than2% would be needed to achieve greater than approximately 2.5 GPa stressin the wires.

In another example, FIG. 11B is a plot 1100B depicting stress simulationof the structure shown in FIG. 3B, in accordance with an embodiment ofthe present invention. Referring to plot 1100B, the compressivelystrained GeSn layers cause the Ge to be stretched out as theyelastically relax causing tensile Ge. In this case, the GeSn has 2%compressive strain to begin with, as grown on a virtual substrate. Thetwo in plane stresses (in dynes/cm₂) are shown for the Ge layers only.It is to be understood that higher stresses may be achieved byincreasing the mismatch to the virtual substrate. One option may be touse relaxed SiGe virtual substrates instead of Ge. Such an approach maybe needed for the smallest wire dimensions.

In another example, FIG. 11C is a plot 1100C depicting stress simulationof the structure shown in FIG. 3C, in accordance with an embodiment ofthe present invention. Referring to plot 1100C, this approach results inlarge tensile stresses of greater than approximately 2.5 GPa at thesource/channel interface. This may allow the use of lower Snconcentrations in the GeSn layers.

In the above described embodiments, whether formed on virtual substratelayers or on bulk substrates, an underlying substrate used for TFETdevice manufacture may be composed of a semiconductor material that canwithstand a manufacturing process. In an embodiment, the substrate is abulk substrate, such as a P-type silicon substrate as is commonly usedin the semiconductor industry. In an embodiment, substrate is composedof a crystalline silicon, silicon/germanium or germanium layer dopedwith a charge carrier, such as but not limited to phosphorus, arsenic,boron or a combination thereof. In one embodiment, the concentration ofsilicon atoms in the substrate is greater than 97% or, alternatively,the concentration of dopant atoms is less than 1%. In anotherembodiment, the substrate is composed of an epitaxial layer grown atop adistinct crystalline substrate, e.g. a silicon epitaxial layer grownatop a boron-doped bulk silicon mono-crystalline substrate.

The substrate may instead include an insulating layer disposed inbetween a bulk crystal substrate and an epitaxial layer to form, forexample, a silicon-on-insulator substrate. In an embodiment, theinsulating layer is composed of a material such as, but not limited to,silicon dioxide, silicon nitride, silicon oxy-nitride or a high-kdielectric layer. The substrate may alternatively be composed of a groupIII-V material. In an embodiment, the substrate is composed of a III-Vmaterial such as, but not limited to, gallium nitride, galliumphosphide, gallium arsenide, indium phosphide, indium antimonide, indiumgallium arsenide, aluminum gallium arsenide, indium gallium phosphide,or a combination thereof. In another embodiment, the substrate iscomposed of a III-V material and charge-carrier dopant impurity atomssuch as, but not limited to, carbon, silicon, germanium, oxygen, sulfur,selenium or tellurium.

In the above embodiments, TFET devices include source drain regions thatmay be doped with charge carrier impurity atoms. In an embodiment, thegroup IV material source and/or drain regions include N-type dopantssuch as, but not limited to phosphorous or arsenic. In anotherembodiment, the group IV material source and/or drain regions includeP-type dopants such as, but not limited to boron.

In the above embodiments, although not always shown, it is to beunderstood that the TFETs would further include gate stacks. The gatestacks include a gate dielectric layer and a gate electrode layer. In anembodiment, the gate electrode of gate electrode stack is composed of ametal gate and the gate dielectric layer is composed of a high-Kmaterial. For example, in one embodiment, the gate dielectric layer iscomposed of a material such as, but not limited to, hafnium oxide,hafnium oxy-nitride, hafnium silicate, lanthanum oxide, zirconium oxide,zirconium silicate, tantalum oxide, barium strontium titanate, bariumtitanate, strontium titanate, yttrium oxide, aluminum oxide, leadscandium tantalum oxide, lead zinc niobate, or a combination thereof.Furthermore, a portion of gate dielectric layer may include a layer ofnative oxide formed from the top few layers of the corresponding channelregion. In an embodiment, the gate dielectric layer is composed of a tophigh-k portion and a lower portion composed of an oxide of asemiconductor material. In one embodiment, the gate dielectric layer iscomposed of a top portion of hafnium oxide and a bottom portion ofsilicon dioxide or silicon oxy-nitride.

In an embodiment, the gate electrode is composed of a metal layer suchas, but not limited to, metal nitrides, metal carbides, metal silicides,metal aluminides, hafnium, zirconium, titanium, tantalum, aluminum,ruthenium, palladium, platinum, cobalt, nickel or conductive metaloxides. In a specific embodiment, the gate electrode is composed of anon-workfunction-setting fill material formed above a metalworkfunction-setting layer. In an embodiment, the gate electrode iscomposed of a P-type or N-type material. The gate electrode stack mayalso include dielectric spacers.

The TFET semiconductor devices described above cover both planar andnon-planar devices, including gate-all-around devices. Thus, moregenerally, the semiconductor devices may be a semiconductor deviceincorporating a gate, a channel region and a pair of source/drainregions. In an embodiment, semiconductor device is one such as, but notlimited to, a MOS-FET. In one embodiment, semiconductor device is aplanar or three-dimensional MOS-FET and is an isolated device or is onedevice in a plurality of nested devices. As will be appreciated for atypical integrated circuit, both N- and P-channel transistors may befabricated on a single substrate to form a CMOS integrated circuit.Furthermore, additional interconnect wiring may be fabricated in orderto integrate such devices into an integrated circuit.

Generally, one or more embodiments described herein are targeted attunneling field effect transistors (TFETs) for CMOS architectures andapproaches to fabricating N-type and P-type TFETs. Group IV activelayers for such devices may be may be formed by techniques such as, butnot limited to, chemical vapor deposition (CVD) or molecular beamepitaxy (MBE), or other like processes.

FIG. 12 illustrates a computing device 1200 in accordance with oneimplementation of the invention. The computing device 1200 houses aboard 1202. The board 1202 may include a number of components, includingbut not limited to a processor 1204 and at least one communication chip1206. The processor 1204 is physically and electrically coupled to theboard 1202. In some implementations the at least one communication chip1206 is also physically and electrically coupled to the board 1202. Infurther implementations, the communication chip 1206 is part of theprocessor 1204.

Depending on its applications, computing device 1200 may include othercomponents that may or may not be physically and electrically coupled tothe board 1202. These other components include, but are not limited to,volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flashmemory, a graphics processor, a digital signal processor, a cryptoprocessor, a chipset, an antenna, a display, a touchscreen display, atouchscreen controller, a battery, an audio codec, a video codec, apower amplifier, a global positioning system (GPS) device, a compass, anaccelerometer, a gyroscope, a speaker, a camera, and a mass storagedevice (such as hard disk drive, compact disk (CD), digital versatiledisk (DVD), and so forth).

The communication chip 1206 enables wireless communications for thetransfer of data to and from the computing device 1200. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The communication chip 1206 may implementany of a number of wireless standards or protocols, including but notlimited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE,GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well asany other wireless protocols that are designated as 3G, 4G, 5G, andbeyond. The computing device 1200 may include a plurality ofcommunication chips 1206. For instance, a first communication chip 1206may be dedicated to shorter range wireless communications such as Wi-Fiand Bluetooth and a second communication chip 1206 may be dedicated tolonger range wireless communications such as GPS, EDGE, GPRS, CDMA,WiMAX, LTE, Ev-DO, and others.

The processor 1204 of the computing device 1200 includes an integratedcircuit die packaged within the processor 1204. In some implementationsof the invention, the integrated circuit die of the processor includesone or more devices, such as tunneling field effect transistors (TFETs)built in accordance with implementations of the invention. The term“processor” may refer to any device or portion of a device thatprocesses electronic data from registers and/or memory to transform thatelectronic data into other electronic data that may be stored inregisters and/or memory.

The communication chip 1206 also includes an integrated circuit diepackaged within the communication chip 1206. In accordance with anotherimplementation of the invention, the integrated circuit die of thecommunication chip includes one or more devices, such as tunneling fieldeffect transistors (TFETs) built in accordance with implementations ofthe invention.

In further implementations, another component housed within thecomputing device 1200 may contain an integrated circuit die thatincludes one or more devices, such as tunneling field effect transistors(TFETs) built in accordance with implementations of the invention.

In various implementations, the computing device 1200 may be a laptop, anetbook, a notebook, an ultrabook, a smartphone, a tablet, a personaldigital assistant (PDA), an ultra mobile PC, a mobile phone, a desktopcomputer, a server, a printer, a scanner, a monitor, a set-top box, anentertainment control unit, a digital camera, a portable music player,or a digital video recorder. In further implementations, the computingdevice 1200 may be any other electronic device that processes data.

Thus, embodiments of the present invention include tunneling fieldeffect transistors (TFETs) for CMOS architectures and approaches tofabricating N-type and P-type TFETs.

In an embodiment, a tunneling field effect transistor (TFET) includes ahomojunction active region disposed above a substrate. The homojunctionactive region includes a relaxed Ge or GeSn body having an undopedchannel region therein. The homojunction active region also includesdoped source and drain regions disposed in the relaxed Ge or GeSn body,on either side of the channel region. The TFET also includes a gatestack disposed on the channel region, between the source and drainregions. The gate stack includes a gate dielectric portion and gateelectrode portion.

In one embodiment, the relaxed Ge or GeSn body is a direct band gap bodyand has a thickness of, or less than, approximately 5 nanometers.

In one embodiment, the TFET is a finfet, trigate or squarenanowire-based device.

In one embodiment, the doped source and drain regions include N-typedopants and the TFET is an N-type device.

In one embodiment, the doped source and drain regions include P-typedopants and the TFET is a P-type device.

In an embodiment, a tunneling field effect transistor (TFET) includes ahetero-junction active region disposed above a substrate. Thehetero-junction active region includes a relaxed body having a Ge orGeSn portion and a lattice matched Group III-V material portion andhaving an undoped channel region in both the Ge or GeSn portion and thelattice matched Group III-V material portion. A doped source region isdisposed in the Ge or GeSn portion of the relaxed body, on a first sideof the channel region. A doped drain region is disposed in the GroupIII-V material portion of the relaxed body, on a second side of thechannel region. The TFET also includes a gate stack disposed on thechannel region, between the source and drain regions. The gate stackincludes a gate dielectric portion and gate electrode portion.

In one embodiment, the Ge or GeSn portion of the relaxed body is a Geportion, and the lattice matched Group III-V material portion is a GaAsor Ga_(0.5)In_(0.5)P portion.

In one embodiment, the relaxed body is a direct band gap body and has athickness of, or less than, approximately 5 nanometers.

In one embodiment, the TFET is a finfet, trigate or squarenanowire-based device.

In one embodiment, the doped source and drain regions include N-typedopants and the TFET is an N-type device.

In one embodiment, the doped source and drain regions include P-typedopants and the TFET is a P-type device.

In an embodiment, a tunneling field effect transistor (TFET) includes ahomojunction active region disposed above a relaxed substrate. Thehomojunction active region includes a biaxially tensile strained Ge orGe_(1-y)Sn_(y) body having an undoped channel region therein. Dopedsource and drain regions are disposed in the biaxially tensile strainedGe or Ge_(1-y)Sn_(y) body, on either side of the channel region. TheTFET also includes a gate stack disposed on the channel region, betweenthe source and drain regions. The gate stack includes a gate dielectricportion and gate electrode portion.

In one embodiment, the relaxed substrate is a Ge_(1-x)Sn_(x) (x>y) orIn_(x)Ga_(1-x)As substrate.

In one embodiment, the biaxially tensile strained Ge or Ge_(1-y)Sn_(y)body is a direct band gap body and has a thickness of, or less than,approximately 5 nanometers.

In one embodiment, the TFET is a planar, finfet, trigate or squarenanowire-based device.

In one embodiment, the TFET is a finfet or trigate device, with strainedGe or Ge_(1-y)Sn_(y) body with uniaxial tensile stress along a crystalorientation of <100>, <010> or <001>.

In one embodiment, the doped source and drain regions include N-typedopants and the TFET is an N-type device.

In one embodiment, the doped source and drain regions include P-typedopants and the TFET is a P-type device.

In an embodiment, a tunneling field effect transistor (TFET) includes ahetero-junction active region disposed above a substrate. Thehetero-junction active region includes a vertical nanowire having alower Ge portion and an upper GeSn portion and having an undoped channelregion in only the GeSn portion. A doped source region is disposed inthe Ge portion of the vertical nanowire, below the channel region. Adoped drain region is disposed in the GeSn portion of the verticalnanowire, above the channel region. The TFET also includes a gate stackdisposed surrounding the channel region, between the source and drainregions. The gate stack includes a gate dielectric portion and gateelectrode portion.

In one embodiment, the lower Ge portion of the vertical nanowire isdisposed on a virtual substrate portion of the substrate, and thevirtual substrate is a relaxed InGaAs or relaxed GeSn virtual substrate.

In one embodiment, the lower Ge portion of the vertical nanowire isdisposed on a compressively strained GeSn layer.

In one embodiment, the lower Ge portion of the vertical nanowire isdisposed on a larger Ge region disposed on a virtual substrate portionof the substrate, and the virtual substrate is a relaxed GeSn virtualsubstrate.

In one embodiment, the GeSn virtual substrate is composed ofapproximately 14% Sn, and the upper GeSn portion of the verticalnanowire is compressively strained and is composed of approximately 28%Sn.

In one embodiment, the lower Ge portion has tensile strain.

In one embodiment, from a top-down perspective, the vertical nanowirehas an approximately square geometry, and the tensile strain is abiaxial tensile strain.

In one embodiment, the lower Ge portion has a vertical dimensionapproximately in the range of 2-4 nanometers.

In one embodiment, the doped source and drain regions include N-typedopants and the TFET is an N-type device.

In one embodiment, the doped source and drain regions include P-typedopants and the TFET is a P-type device.

In an embodiment, a tunneling field effect transistor (TFET) includes ahetero-junction active region disposed above a substrate. Thehetero-junction active region includes a vertical nanowire having alower tensile strained Ge_(1-y)Sn_(y) portion and an upperGe_(1-x)Sn_(x) portion and having an undoped channel region in only theGe_(1-x)Sn_(x) portion, where x>y. A doped source region is disposed inthe Ge_(1-y)Sn_(y) portion of the vertical nanowire, below the channelregion. A doped drain region is disposed in the Ge_(1-x)Sn_(x) portionof the vertical nanowire, above the channel region. A gate stack isdisposed surrounding the channel region, between the source and drainregions. The gate stack includes a gate dielectric portion and gateelectrode portion.

In one embodiment, the lower tensile strained Ge_(1-y)Sn_(y) portion ofthe vertical nanowire is disposed on a virtual substrate portion of thesubstrate, and the virtual substrate is a relaxed InGaAs or relaxed GeSnvirtual substrate.

What is claimed is:
 1. An integrated circuit structure, comprising: asource structure above a substrate, the source structure comprisinggermanium, wherein the source structure comprises an upper narrowportion on a lower wide portion, the lower wide portion having asubstantially flat surface around the upper narrow portion; a channelstructure on the upper narrow portion of the source structure; a drainstructure on the channel structure, the drain structure comprisinggermanium and tin; and a gate electrode at least partially surroundingthe channel structure.
 2. The integrated circuit structure of claim 1,wherein the source structure has tensile strain.
 3. The integratedcircuit structure of claim 1, wherein the channel structure hascompressive strain, and wherein the drain structure has compressivestrain.
 4. The integrated circuit structure of claim 1, wherein thechannel structure comprises germanium and tin, wherein the channelstructure comprises approximately 28% tin, and wherein the drainstructure comprises approximately 28% tin.
 5. The integrated circuitstructure of claim 1, wherein the substrate comprises germanium and tin.6. The integrated circuit structure of claim 5, wherein the substrate isa relaxed substrate.
 7. The integrated circuit structure of claim 5,wherein the substrate comprises approximately 14% tin.
 8. The integratedcircuit structure of claim 1, wherein the gate electrode completelysurrounds the channel structure.
 9. The integrated circuit structure ofclaim 1, further comprising: a gate dielectric layer between the gateelectrode and the channel structure.
 10. The integrated circuitstructure of claim 1, wherein the integrated circuit structure is avertical tunneling field effect transistor (vertical TFET) device. 11.An integrated circuit structure, comprising: a source structure above asubstrate, the source structure comprising germanium and tin, whereinthe source structure has an approximately square geometry from atop-down perspective above the substrate; a channel structure on thesource structure, the channel structure having the approximately squaregeometry from the top-down perspective above the substrate; a drainstructure on the channel structure, the drain structure comprisinggermanium and tin, and the drain structure having the approximatelysquare geometry from the top-down perspective above the substrate; and agate electrode at least partially surrounding the channel structure. 12.The integrated circuit structure of claim 11, wherein the channelstructure comprises germanium and tin.
 13. The integrated circuitstructure of claim 11, wherein the source structure has tensile strain.14. The integrated circuit structure of claim 13, wherein the tensilestrain is a biaxial tensile strain.
 15. The integrated circuit structureof claim 11, wherein the substrate comprises germanium and tin.
 16. Theintegrated circuit structure of claim 15, wherein the substrate is arelaxed substrate.
 17. The integrated circuit structure of claim 11,wherein the substrate comprises indium, gallium and arsenic.
 18. Theintegrated circuit structure of claim 17, wherein the substrate is arelaxed substrate.
 19. The integrated circuit structure of claim 11,wherein the gate electrode completely surrounds the channel structure.20. The integrated circuit structure of claim 11, further comprising: agate dielectric layer between the gate electrode and the channelstructure.
 21. The integrated circuit structure of claim 11, wherein theintegrated circuit structure is a vertical tunneling field effecttransistor (vertical TFET) device.